Zimmer concludes his book with a chapter discussing macroevolution. He explains although fish and whales both live in the same environment, they have different strategies for doing so. Whales do not swim like fish, but use an adaptation of the mammalian gait in the water. Since whales did not have a fish’s lateral lines to rely on they needed to keep their terrestrial sense organs such as ears in working order. He suggests that the expansion of whales into the ocean was due to the fact that it was a wide open niche. Gould and Eldredge came up with a hypothesis called punctuated equilibrium to describe how macroevolution occurs. This hypothesis says that most adaptations to a species occur as the species is originating, not during its lifetime. It used to be thought that mosasaurs became extinct 65 million years ago without evolving into any new forms. Debraga and Carroll showed that this was not the case. Mosasaurs evolved into amphibious crocodilelike animals over the period of 65 million years. They also found that rate of evolution was slow during the ancestors of mosasaurs increased during their origin and slowed down again after their origin. They also found that the most important adaptations to water occurred long after the mosasaurs became committed to living in the water.